Kelp Forests: Cathedrals in the Sea

Pyjama Catshark

Due to its nocturnal habits, the beautiful Pyjama Catshark (Poroderma
africanum) is rarely seen in the wild. Endemic to southern African
waters, it is most often encountered by shore-based night divers as they
slither across beds of Eklonia kelp to reach deeper water. Sporting a
hide of iridescent bronze marked with five to seven thick, chocolate-brown
longitudinal stripes, this smallish, attractive species is skittish of
divers and quite harmless toward them.

The Pyjama Catshark is fairly abundant throughout its range. Due to this
species’ attractive markings, relatively small size, tolerance of being
shipped via air freight, and ability to adapt well to captivity, it is a
popular exhibit in public aquaria. But little is known about how the Pyjama
Catshark makes a living in its natural environment.

We do know that the Pyjama Catshark rests in caves during daylight hours,
becoming active at night. This species lays rectangular, yellowish-green egg
cases, each measuring about 1 by 4 inches (3 by 10 centimetres), which are
anchored to kelp by long, stringy tendrils at each corner. The Pyjama
Catshark exploits a broad spectrum of food sources. Known prey of this shark
includes teleost fishes — such as anchovies, gurnards, and hakes — as well
as peneid shrimps, mantis shrimps, crabs, squids, octopuses, cuttlefishes,
bivalves, and polychaete worms. It also opportunistically scavenges fish
offal. The Pyjama Catshark’s ability to take advantage of so many different
foods undoubtedly contributes to its abundance in near-shore waters.

Near-shore waters are subject to wide fluctuations in salinity due to
river run-off and coastal precipitation. Such variability in dissolved salt
concentration demands that coastal creatures be able to passively tolerate
fluctuations of their internal salt-to-water balance or actively regulate
their internal saltiness. Like other sharks, the Pyjama Catshark maintains
an internal osmotic pressure slightly higher than that of the surrounding
medium — a condition known as “hyper-osmotic” — by concentrating nitrogenous
wastes (principally urea and TMAO) in its tissues.

A classic series of experiments conducted on captive Pyjama Catsharks
tested this species’ ability to osmoregulate. Two groups of similar-sized
test sharks were exposed to the same range of salinities. A “high intake”
group received a given quantity of food twice a week; a “low intake” group
received the same quantity of food only once a month. Measurements of blood
osmolarity (total osmotic pressure), chloride, and urea concentrations
indicated that the “high intake” group maintained the slightly hyper-osmotic
profile characteristic of sharks, while the “low intake” group’s blood
parameters were distinctly hypo-osmotic. When previously “high intake”
sharks were starved over a period of one month, they showed the same
response to fluctuating salinities as did the “low intake” group. Because
urea and TMAO are waste products of protein metabolism, it stands to reason
that well-fed sharks would have higher tissue concentrations of these wastes
and thus a higher overall osmotic pressure. From these results, the
researcher concluded that a Pyjama Catshark’s ability to osmoregulate
depends upon its feeding success.

To ensure feeding success, Pyjama Catsharks opportunistically exploit any
abundant food source and employ some remarkable predatory strategies. One of
the most amazing of these was only recently discovered.

Pyjama Catsharks have a particular gastronomic fondness for squid. Squids
are fast and agile jet-propelled molluscs that school, change color and
patterns continually, squirt inky camouflage, have keen eyesight, and can
quickly learn to distinguish a shark from a non-threatening teleost. This
combination of abilities makes squids very difficult for sharks to catch.
Some species of squid mate at night and die immediately after spawning,
creating rich pickings for scavengers (as described in the Horn Shark
profile, above). But the Chokka Squid (Loligo reynauldii) of southern
Africa is less obliging, typically mating during daylight hours and jetting
away at high speed after spawning. These squids must therefore be hunted
actively and Pyjama Catsharks have developed a clever strategy for doing so.

Recently, breeding behavior of the Chokka Squid was filmed by researchers
in Tsitsikamma Coastal National Park, South Africa. Here, the peak breeding
season of Chokka Squid occurs from October through December, during which
these squids congregate to mate and lay their eggs in huge, communal beds.
At the researchers’ site, more than 300 pairs of Chokka Squid shared 20 egg
beds up to 30 feet (10 metres) apart. As the female squid uses her arms to
attach strands of fertilized eggs to the sea floor, her mate hovers nearby
to ward off rival males. To take advantage of this predictable feeding
opportunity, normally nocturnal Pyjama Catsharks venture forth in broad
daylight to the Chokka Squids’ offshore mating grounds and hide motionless
in their egg beds. Among the squid egg cases, the researchers noted 20
Pyjama Catsharks lying in wait with their heads hidden and striped bodies
protruding.

The attractive stripes that adorn Pyjama Catsharks help to break up their
outline into unrecognizable shapes. This camouflage enables these sharks to
hide among Chokka Squid egg strands, putting them in a perfect position to
ambush the distracted squids from close range. At Tsitsikamma, the
researchers actually observed several Pyjama Catsharks lunge suddenly from
their hiding places and attempt to capture female squids as they affixed
their strands of eggs. Although at least one squid was grasped and severely
injured by an ambushing Pyjama Catshark, no successful predation by the
sharks was witnessed. Nevertheless, the ‘squid-mugging’ strategy of Pyjama
Catsharks seems likely to enable these adaptable little sharks to take
advantage of a rich, seasonally available food source.